Dopamine (DA) neurotransmission in the striatum is implicated in a broad range of behaviors and disorders. Understanding presynaptic mechanisms that regulate DA transmission and their consequences on corticostriatal transmission is important for understanding the synaptic basis of drug abuse and addiction, and habit and motor learning. This proposal is a competing continuation intended to elucidate presynaptic mechanisms underlying plasticity of DAergic neurotransmission and its effects on synaptic computation in the striatum. Efforts to date by our lab and others on characterizing striatal DA transmission have monitored combined release from hundreds to thousands of synapses. Here we describe a novel optical approach that will extend previous investigations by introducing "false fluorescent neurotransmitters" that provide the first means to directly visualize neurotransmitter release, doing so by monitoring the presynaptic activity of individual striatal DA terminals, allowing subpopulations to be identified. We outline direct means to answer long-asked questions on synaptic modulation of DA release, a highly controversial area, and how psychostimulant drugs interfere with these processes. These properties must be characterized to understand how DA regulates habit learning and its role in addiction. The aims and underlying hypotheses of this proposal are: 1) Identify regional and activity-dependent mechanisms in DA presynaptic regulation. Hypothesis: DA terminals experience ongoing feedback regulation by presynaptic receptors, including D2 DA autoreceptors and nACh and mGlu heteroreceptors that act as high-pass filters to provide focal DA input. 2) Determine how individual presynaptic DA terminals regulate the kinetics of vesicle fusion to modulate terminal state. Hypothesis: "Flickering" fusion associatedwith ongoing tonic activity converts to "full" fusion during burst firing to provide focal DA input. 3) Define how focal DA input selects particular corticostriatal terminals. Hypothesis: Enhanced signal/background of DA input regulated by the mechanisms above selects nearby sets of corticostriatal inputs.